Air bags are inflatable bags that remain folded and out of sight in readiness for a frontal collision. During a collision, chemical reaction of a material typically containing sodium azide produces gaseous products which inflate the bag and interpose it between the driver and the steering wheel or the front seat occupant and the dashboard.
A sensor of the type to which this invention is applicable is marketed by Breed Automotive Corp. It has a metallic ball free to move in a sealed cylinder. Air flow around the ball causes a pressure differential. The pressure differential causes a force proportional to and opposite the relative velocity of the ball with respect to the cylinder. The proportionality of force to velocity makes the sensor an acceleration integrator that initiates a chemical reaction upon achievement of a predetermined velocity change. The variation of air viscosity with temperature is compensated by making the cylinder and ball of materials of different thermal expansion coefficients thereby causing the gap between the ball and cylinder to change with temperature as required to maintain the performance of the sensor over a wide temperature range. A firing mechanism for initiating combustion of a stab primer includes a pin propelled by a first spring into the stab primer. A release means for the firing mechanism includes a trigger in the form of a metal rod extending perpendicular to a pivoted shaft having a "D" shaped cross section. The trigger is moved by the ball to its firing position during a crash. A second spring urges the trigger away from the firing position thereby causing the trigger to maintain the ball in a normal or resting position during normal operation of the vehicle. The force the trigger applies to the ball modifies the pure velocity change character of the sensor so that the velocity change required for release of the firing pin increases with the duration of the deceleration pulse of the crash. Movement of the ball during a crash rotates the trigger until the flat portion of the D-shaft releases the firing pin which is thereupon driven by the spring into a stab primer. Ignition of the stab primer initiates combustion of larger amounts of propellent material that generates gas for filling the air bag. Two complete sensor and firing mechanisms are combined into one package to enhance reliability by providing two units each capable of initiating combustion of the propellant.
This sensor is expensive to manufacture and one reason for the high cost is the large number of complex parts and the precision required in the trigger mechanism. Great care is required to make each part so as to insure minimum friction and maximum precision during the life of the sensor.
A snap disk is a piece of spring material that has a normal relaxed shape which approximates a section of a sphere. In this shape its potential energy is at its lowest value. It is also in equilibrium, or will also remain indefinitely in a second shape in which it is dished in the opposite direction. In its second shape it has greater potential energy than in its normal relaxed shape and will energetically snap into its relaxed position when it is urged in that direction by force applied to its center.
A stab primer is a commercially available unit containing of a crystal that ignites when it is cracked. The crystal in sealed in a container having a cover plate positioned adjacent to a face of the crystal and designed to be pierced by a pointed object. Piercing the cover plate by a pin having sufficient energy ignites the crystal which initiates combustion of an ignition mix which in turn ignites the propellent material for producing gas for filling an air bag.
A general object of this invention is to provide a crash sensor for automotive vehicles which also overcomes certain disadvantages of the prior art.